Follow-up Actions Chernobyl Unit 4

After the 1986 accident at the Unit 4 at the Chernobyl site, Ukrainian and Russian authorities constructed a new “containment" for the damaged reactor. This structure (the Ukritiye) was erected in great haste and under extremely arduous conditions. Subsequent monitoring and analysis of the Ukritiye has shown that the initial expectation of a 30 year lifetime was unlikely to be achieved.

The Ukrainian authorities in 1992 initiated a competition of ideas for the safe enclosure of the damaged unit 4 at Chernobyl (Kiev ’92 competition). Following the Kiev competition, the Commission of the European Union financed a feasibility study for the safe enclosure of Unit 4. This work was carried out by the European consortium "Alliance" which reported in July 1995. The study compiled and verified many existing data relating to Chernobyl, evaluated the options for safe enclosure and presented a recommended design for a structure (see project 22721 and 23141).

The conclusions and recommendations of the Alliance study were based substantially on the recommendations of the jury of the Kiev '92 competition and were supported by the Ukrainian Authorities. However, in 1996 some conclusions developed by Alliance did not reflect the real situation at the site anymore (Unit 1,2,3 back to operation, etc.).

Therefore this contract was awarded to the Alliance Consortium in the framework of the development of a Shelter Implementation Plan (SIP) for Chernobyl, managed by Trischler und Partner (TUP) (see project 23829). This contract had to update the previous Alliance study developed for the Kiev ‘92, redefine the safety objectives, and to identify the design criteria that meet these safety objectives. An integral part of the present study was the cost benefit analysis which would ensure that the solution finally recommended would meet the 'least cost' criterion that is required by most of the international financial institutes that might, ultimately, fund the project.

This contract was awarded to Alliance to provide an essential technical support and source of background information to the Project Manager and Co-ordinator (TuP).

The Contractor objective was to provide TUP with the necessary information that had been acquired during the previous study, to arrange for the participation of the key Ukrainian Institutes and Organizations that had been identified during the previous study, and to use its experts to contribute to the preparation of the design criteria and to the development of appropriate solutions.

The project was organised in the following steps:

• Recovery of available project data, updating of some technical reports developed by Ukrainian Institutions.
• Issuing of contracts to Ukrainian Institutes: NIISK (existing conditions, seismicity, stabilisation), ISTC (fire, fuel containing masses, contamination), Interatominstrument (monitoring equipment), Atomenergostrojproekt (rapid response of the dust suppression system), VNIPIET, St. Petersburg (entombment option for long term measures).
• Definition of a reference set of scenario for the detailed analysis of the possible solutions to the remediation of the Ukritiye. The following scenarios were identified:
  • Scenario type 1: “Do nothing”, except organisational measures. This scenario w as selected for comparison purposes.
  • Scenario type 2: implement immediate Short Term Measures and continue them in future as long term measure. This scenario implements a system for risk management but does not solve the problem in the future.
  • Scenario type 3: implement immediate Short Term Measures and a shelter in preparation of a soon removal. This shelter must face the containing requirements as long as the removal of radioactive inventory begins. This scenario may also be considered for deferred removal. In this case, there would be two options: early (3a) and deferred (3b) removal within Scenario type 3.
  • Scenario type 4: implement immediate short term measures and shield the radioactive inventory with bulk material, e. g. sand, in order to optimise and to stabilise the present Ukritiye before implementing a light shelter for confinement and for an early or deferred removal.
  • Scenario type 5: implement immediate short term measures and shield the radioactive inventory with cementitious fill in order to optimise and stabilise the present Ukritiye, before implementing a light shelter for confinement (if needed) and to proceed to early or deferred removal.
• Development of design criteria: this task was covered by a separate contract to a western regulatory Body. However, the contractor prepared a statement of the key safety principles that would underpin the safety objectives.
• Development of Scenario n.2, 3b and 5. The following was analysed:
  • The work necessary to improve the existing monitoring equipment of the state of the Ukritiye, including the condition of the structure and foundations, seismicity at the site and in the region, the fuel containing masses (including criticality monitoring), the water level inside and outside the structure, the risk of fire and the spread of radioactive contamination.
  • The work necessary to prevent, as far as possible, the release of radioactive material (e.g. through stabilisation).
  • The work necessary to contain the release of radioactive material following a structural failure or any other event leading to such release.
• Technical review and integration of the additional studies and information made available by some US consultants (e.g. on the criticality of the left over fuel, etc.).
• Cost analysis for all the scenarios, including the monitoring of the Ukritiye, the retrieval of waste and its disposal.

The study identified the following main issues to be solved for a successful cleaning project and provided proposals for their management.

4) Radioactive waste disposal

The costs of the disposal of the radioactive waste from the Ukritiye will dominate all decisions relating to the management of the Chernobyl site. The existing Ukritiye and its surroundings alone amount to some 500,000 cubic meter of radioactive waste, which will require treatment and disposal under strictly controlled conditions. The actual disposal method will depend on the classification of the individual waste packages, and the efficiency of the waste sorting and sentencing will have a substantial effect on the costs. Nevertheless, the total costs for the disposal of these wastes, based on a reasonable assessment of the management technology, will be of the order of billion dollars.
It is therefore essential, in order to minimise the problems for future generations, that the total volume of material involved in any remedial works at the Chernobyl site should be minimised, since all constructions will, to some extent, ultimately become radioactive waste themselves. A delay of about 50 years will allow the heat output from the fuel to decay, so that a specific management route for High Level Waste (HLW) is no more necessary. However, the cost saving is small compared with the cost of dealing with the vast amount of Low and Intermediate Level Waste (LILW) that exists. If a further delay, of some 500 years, is allowed, then the short lived waste (LILW-SL) will have decayed completely and will require a simple management, without remote handling technology, and disposal. The cost savings thus incurred are substantial, ie. 50%: however, there is a serious risk that, if the removal is delayed too long, effective waste sorting will not be possible in practice and all the wastes will have to be classified as long lived (LILW-LL), with an enormous increase in costs (200%).

5) Removal of fuel containing masses

There are two predominant arguments for the removal of the fuel and the fuel containing masses from the Ukritiye.

The first is to satisfy public opinion over the possibility of a criticality event: expert assessments of this have shown that the risk of criticality could only increase by changing the physical conditions of the fuel containing masses. This could involve, for example, the collapse of the internal structures, flooding or major changes in the physical and chemical state of the FCM itself. The problem of flooding is simply, and already, managed by the use of neutron absorbers. Since the fuel now resides in the massive sub-reactor regions of the building, structural collapse leading to a concentration of the masses is inconceivable. If the pessimistic view prevails, that is that the risk of a critical excursion can not be excluded, then it must be recalled that the radiological consequences of such an event would be less than those following a collapse of the Ukritiye.
The second is to reduce the inventory of radioactive material which can be ultimately released to the biosphere in the event of failure of the containment. This problem can be addressed by other means at a lower cost both in cash and in radiation dose to the workers involved.
The considered opinion of the experts on this study is that there is no compelling technical reason for the prompt removal of the fuel. Nevertheless, an estimation by a US group determined that the cost of immediate removal of the fuel and fuel containing masses would be of the order of hundred million dollars and, moreover, would involve a substantial, and risky, programme of development of the appropriate technology.

The risk to the workforce, arising from the radiation dose concerned, on such a project would be also very high and the overall reduction in risk to the population at large, small.

6) Removal of inventory

The dismantling and removal of the Ukritiye, and its immediate environs, gives rise to the greatest volume of radioactive waste. It will therefore be the most expensive stage of the remediation. The radiation dose burden to the workers will depend on the technology and the timing of the operation: more expensive technology and a delayed dismantling will reduce the dose.
The timing of the dismantling is also driven by the existence of a radioactive waste repository in Ukraine. In the absence of such a repository, it will be necessary to construct an interim waste storage facility. There is thus a clear benefit in dose budget, in cash flow and in overall cost in deferring the dismantling and removal of the Ukritiye.

Given the cost of the radioactive waste disposal that would be incurred by removal of the wastes, it is worth considering the costs and risks associated with indefinite storage of the material at the Chernobyl site.

7) Duration of containment

The durability, or life span, of the new containment is dependent upon the decision taken for the timing of the dismantling of the Ukritiye. An early dismantling will allow the construction of a shelter of a short life-span. Against this cost saving, must be considered the risk that the dismantling may be delayed beyond the design life of the shelter.

If the decision is taken to defer the dismantling, then the new shelter must be of sufficient size and robustness to protect, not only the Ukritiye, but the surrounding buried radioactive material including the Pioneer Wall. The cost will then reflect both the increased durability and the increased size.

8) Stabilisation of the Ukritiye

The study has evaluated the various measures necessary to reduce the risk and consequences of a collapse of the present Ukritiye. These fall into three broad groups: monitoring of the condition, minimisation of the consequences of collapse (mainly dust suppression systems) and stabilisation of the existing structure.

The works proposed by the responsible Ukrainian body, NIISK, for stabilisation involve a dose budget of 2000 man.Sv. Such a total dose is by any standards unacceptable, and to make the individual dose reasonable would require a workforce of some 40,000.

Therefore, the following strategy was proposed:

• If the cash flow sufficient for the construction of a shelter and the provision of a radioactive waste repository can be assured, then the construction of a minimum containment and immediate dismantling should be considered. Cost: $1010, risk/dose: short term, high; long term, low.
• If the cash flow can not be assured for the entire process, then a shelter must be constructed capable of containing the existing Ukritiye and allowing a deferred dismantling and removal. This solution will allow the flexibility for the government of Ukraine to decide to deal with the site when it considers appropriate, having regard to the political and financial context and to the existence of appropriate radioactive waste disposal facilities. Cost: $1010, risk/dose: short term, moderate; long term, low.
• In the absence of an assured cash flow sufficient for the construction of a shelter and an assured radioactive waste repository, the solution may be to consider in situ entombment. Cost: $108, risk/dose: short term, low; long term, specific risk of the need to dismantle and manage the waste at a cost of $1010.

Comments

(quality of the results, lesson learnt, recommendations for follow-up)

The project met the objectives stated in the TORs.

The management of the project was very sound and essential and invaluable support was provided to the Project Manager of the Chernobyl project.

Further Information

Further information on the project results could be sought from the beneficiary organizations.

The Final Report is available at the DG JRC-IET archive.